1. Field of the Invention
The present invention relates to a light emitting device using a light emitting element with a film containing an organic compound that emits light with application of electric field (hereinafter referred to as organic compound layer), as well as an anode and a cathode. Specifically, the present invention relates to a light emitting device using a light emitting element with lower drive voltage than before and longer lifetime. The term “light emitting device” in this specification refers to an image display device or a light emitting device that employs as a light emitting element a light emitting element. Also included in the definition of the light emitting device are a module in which a connector, such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package), is attached to an organic light emitting element, a module in which a printed wiring board is provided on the tip of a TAB tape or a TCP, and a module in which an IC (integrated circuit) is mounted directly to an organic light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
Light emitting elements are now the focus for next generation flat panel display elements due to their thinness, light weight, high speed response, direct current low voltage drive, and other properties. Further, the light emitting elements are self light emitting, have a wide field of view, and therefore have relatively good visibility. They are considered to be effective as elements used in the display screens of electronic equipment, and are being vigorously developed.
The light emitting mechanism of a light emitting element that emits light by the application of an electric field is considered to be one in which, by sandwiching organic compound layers between electrodes and applying a voltage thereto, electrons injected from a cathode recombine with holes injected from an anode within an organic compound layer, at a center of light emission, forming molecular excitons. The molecular excitons then irradiate energy when returning to a ground state, and light is emitted. Note that a singlet excitation state and a triplet excitation state are possible as molecular exciton types formed by the organic compound, and cases of light being emitted by both types of excitation state are included in this specification.
The light emitting elements are divided by driving method into passive matrix (simple matrix) and active matrix types. Note that active matrix types are particularly in the spotlight because high definition display having pixels equal to or greater than QVGA is possible.
An active matrix light emitting device having light emitting elements has an element structure like that shown in FIG. 2. A current control TFT (thin film transistor) 202 is formed on a substrate 201, and an interlayer insulating film 203 is formed on the TFT 202.
An anode (pixel electrode) 205 which is electrically connected to the TFT 202 by a wiring 204 is then formed on the interlayer insulating film 203. Transparent conductive materials having a high work coefficient are suitable as materials for forming the anode 205, and materials such as ITO (indium tin oxide), tin oxide (SnO2), an alloy made from indium oxide and zinc oxide (ZnO), a half transparent film of gold, and polyaniline are proposed. Among these materials, ITO has a band gap of 3.75 eV, and possesses high transparency in the visible light region, and therefore is most often used.
An organic compound layer 206 is formed on the anode 205. Note that all layers formed between an anode and a cathode are defined as the organic compound layer in this specification. Specifically, light emitting layers, hole injecting layers, electron injecting layers, hole transporting layers, electron transporting layers, and the like are included in the organic compound layer 206. Basically, light emitting elements have a structure in which an anode, a light emitting layer, and a cathode are laminated in order. In addition, light emitting elements may also use structures such as one having an anode, a hole injecting layer, a light emitting layer, and a cathode laminated in order, and one having an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, and a cathode laminated in order.
A light emitting element 209 is formed by forming a cathode 207 after forming the organic compound layer 206. Metals having a small work coefficient are often used as cathodes (typically metals residing in group 1 or group 2 of the periodic table). Note that these types of metals (including alkaline metals and alkaline earth metals) are referred to as “alkaline metals” in this specification.
Further, a bank 208 made from an organic resin material is formed so as to cover edge portions of the anode in order to prevent short circuits between the cathode and the anode in the edge portions.
Note that, although only the light emitting element formed in one pixel is shown in FIG. 2, in practice, a plurality thereof are formed in a pixel portion to thereby produce an active matrix type light emitting device.
The anode is an electrode for injecting holes that participate in light emission in the organic compound layer, and it is thought that cracks developing in the anode may adversely influence the generation of holes, may reduce the number of holes injected, and in addition, may become a cause of deterioration of the light emitting element itself. Roughness on an anode surface is thought to exert an adverse influence on the generation and injection of holes, to reduce the number of holes injected, and to cause deterioration of the light emitting element itself due to cracks.
In addition, the organic compound layers easily deteriorate due to oxygen and moisture, and organic resin materials such as polyimide, polyamide, and acrylic are often used as interlayer insulating films. There is a problem in that the light emitting elements deteriorate due to gases such as oxygen generated from interlayer insulating films formed by the aforementioned materials.